In transportation industries, offices and homes, people are often subjected to long periods of sitting. Operators of aircraft, trucks, buses, automobiles, computer terminals, assemblers, office workers, users of wheelchairs and leisure chairs experience difficulty remaining comfortable when seated over extended periods.
For some, discomfort may be only an inconvenience. For most, however, discomfort goes well beyond inconvenience into substantial amounts of lost time off jobs , medical expenses, insurance payments and for some, a necessary change of profession.
In addition to these drawbacks, the costs to industry in fatigue and loss of accuracy in computer terminals, for example, or decreased efficiency and alertness in the transportation industries, may be incalculable.
Numerous devices and means for increasing the comfort level when seated by decreasing the spinal stresses have been devised and are well known in the art. There have been many lumbar support devices that are supposed to aid in maintaining the spinal lordosis. These are mainly singular units that do not account for individual differences. Other devices for head support, variable leg supports and vibration attenuation do not address themselves to the numerous physiological variables of each individual.
Although many of these devices have been useful, these developments have fallen far short of the goal, namely, maximum comfort and efficiency for the seated user.
Human beings are virtually infinitely variable in height, weight, ratios of arm, neck, leg and spinal column lengths and configurations. Maximizing comfort, reducing fatigue and increasing accuracy both on and off the job when seated, requires a close examination of the human body, it's structure and stresses and how these are affected when seated.
The biomechanics of the human spine is quite complex. For seated individuals, the spinal forces are determined by an exacting interplay of spinal geometry, muscle tension, ligament tension, pelvic rotation, and vibration. A common way for expressing this biodynamic system is the measurement of the intradiscal pressure. The greater the disc pressure, the greater is the cumulative strain on key spinal elements.
Additionally, as disc pressure increases, not only is there an increased likelihood of disc fatigue due to the pressure itself but, over time, there is a reduction in the fluid content of the nucleus pulposus (center disc). The healthy nucleus pulposus contains between 70% and 90% fluid which is mostly water. This fluid is essential for distributing the compressive, tensile and shear etressea imposed on the disc.
A key principle in correct biomechanical chair design is to minimize spinal disc force or pressure. This minimization of spinal forces may be brought about by having the seated individual spinal contour partially restored to that which exists when the individual is standing.
Studies have shown that, when one sits down, the spinal lordosis flattens approximately 38 degrees. Approximately two thirds of this flattening is due to the rearward rotation of the pelvis as the ischial tuberosities come in contact with the seat pan. The remaining one third of the spinal flattening is due to the realignment of the vertebral bodies.
One goal of biomechanically correct seating is to have the seated spinal contour matched by the shape of the back rest. Once this is accomplished, increased pressure should be applied to those parts of the spine which have undergone undue flattening in the sitting process.
A second principle of correct biomechanical chair design addressing two thirds of the flattening of the spinal contour is the inclination of the seat pan. Mandal ("The Seated Man",1985) has shown that, by increasing the seat pan downward, you can reduce some of the rearward pelvic rotation relative to the horizontal seat pan. By derotating the pelvis in this manner, you can help restore some of the spinal curvature lost in sitting.
Specifically, the most efficient way to reduce the spinal pressure on the discs is a combination of forward tilting of the seat pan, back rest spinal contour matching and restoration and rearward inclination of the entire back rest. The latter of these promotes a greater sharing of the head, torso and arms between the seat pan and the back rest.
In driving and other work station postures, comfort, practicality and safety limit the adjustability range and benefit of any one of these adjustment features. Therefore, while it is necessary to achieve practical spinal stress reduction and support, it ia necessary to combine the three chair adjustment strategies into one easily adjustable chair.
To bring about the correct adjustment of back rest contour, seat pan inclination and back rest inclination in a practical time interval, it is necessary to automate the seat adjustment mechanism.
The biomechanical goal of the present invention is not only the development of spinal stress reduction and safe supportive seating but to also allow the user to achieve these benefits in the normal course without undue time or effort. It is therefore desirable that the biomechanics of correct seating be combined with an easy-to-use automated method of seat adjustment. The result is not only a biomechanically correct chair but also a seat adjustment system.
Additional biomechanical factors which this chair addresses is the attenuation of spinal vibration through the use of viscoelastic polymers. Shock transmitted through a vehicle to a seat and it's occupant has the effect of reducing fluid content of the spinal nucleus pulposus. Repeated shock and vibration therefore plays an important role in the stress distribution capacity of the spine.
It is well known that bus and truck derivers have among the highest frequencies of back related injuries. In vehicular applications, vibration attenuation is important to maintain the integrity of the spinal structure.
Another biomechanical aspect of correct seating is proper cervical spine support. As the head weighs approximately 7% of total body weight, the forces which may be placed on the cervical disc may be quite extensive. To minimize cervical disc pressure, it is necessary to balance the skull over the atlanto-occipital joint. In doing so, cervical shear and compression forces are minimized.
To achieve the cervical spine stress minimization, it is necessary to maintain cervical spinal lordosis.
As exact spinal configuration may be considered unique for an individual, achievement of lordosis, kyphotic and cervical curves, requires tailoring of a seat's back rest.
Additionally, ingress and egress studies which have been performed have indicated a large percentage of maximum spinal forces needed to enter and leave a vehicle seat. As the mean age of the American population in 1987 is increasing, 6,000,000 endure back pain and injury. An important feature of the biomechanically correct vehicular seat design is the ability of the seat to rotate outward, facing the car door once the door is open. A necessary safety feature for this improvement is an ignition interlock which will not allow the vehicle to start or move with the seat in the rotated, unlocked position.